Eyepiece Selection Help Sheet Magnification = (Telescope Focal Length) / (Eyepiece Focal Length). True-Field of View (TFOV) ~ (Apparent Field of View)/(Magnification), -OR- TFOV ~ (AFOV x Eyepiece Focal Length) / (Telescope Focal Length) NOTE: Both of the above formulas are close approximations. If you know the eyepiece’s field stop, TFOV = (Field Stop) * (57.3) / (Telescope Focal Length) Eye Relief—at least 15mm of eye relief is recommended for eyeglass wearers. Eyeglasses are a necessity if you have astigmatism, especially at low powers. Even without eyeglasses, eye relief under 10mm is a bit tight. Exit Pupil = (Eyepiece Focal Length) / (Telescope f-ratio). Our pupils can dilate up to around 7mm, decreasing to around 5mm as we age. An eyepiece that generates a higher exit pupil than what one's pupil width effectively wastes aperture. EXAMPLE: Suppose we have an 8” f/6 (focal length = 1,200mm) telescope with 30mm, 20mm, 12mm, and 8mm eyepieces. Our magnifications would be 40X (1,200mm/30mm), 60X (1,200mm/20mm), 100X and 150X. Our exit pupils would be 5.0mm (30mm/6), 3.3mm (20mm/6), 2.0mm, and 1.3mm. Suggested exit pupils: 5-7mm: Very low power, typically for maxing out true field of view. 3-5mm: Low power, good for all extended objects 2-3mm: Best overall compromise of magnification, sky contrast and image brightness on a variety of deep-sky objects. Prioritize your eyepiece budget/selection in this range. 1-2mm: Best for small deep-sky objects (particularly planetary nebulae), resolving clusters and planetary work <1mm: Best reserved for steady nights with planets and double stars. Image brightness too low for most nebulae and galaxies. NOTE: If you have a coma corrector, be aware most have an amplification factor when calculating exit pupils. A Televue Paracorr increases your telescope's effective focal length (and, thus, f-ratio) by 15%. A 10" f/4.7 with a Paracorr is effectively a 10" f/5.4. Likewise, an f/10 Schmidt-Cassegrain with a 0.63X focal-reducer acts as an f/6.3 optical system. Also, be aware most focal reducers will often vignette low-power widefield eyepieces, particularly those > 30mm focal length. Focal length selection—this is more of a personal preference, but here are some of the core principles of a good eyepiece line-up: Your “clutch” eyepiece should yield a 2-3mm exit pupil. Buy used to save money. Buy a good barlow lens. Not all nights have steady skies, so let your highest powers (>300X) come via eyepiece + barlow vs. eyepieces alone. A 1.4X ratio between your two most used eyepieces (say, 14mm and 10mm pairing, or a 17mm and 12mm pairing) helps immensely, since a common 2X barlow can extend that ratio two additional focal lengths (14mm, 10mm and 2X barlow = 14mm, 10mm, 7mm and 5mm). Avoid stockpiling low-power eyepieces. FAR more objects are best appreciated with medium/high-powers vs. low-power. I. PLOSSL AND ORTHO EYEPIECES: Televue Plossls—8mm, 11mm, 15mm, 20mm, 25mm, 32mm, 40mm, and 55mm focal lengths. All 1.25", 50°- AFOV except 40mm (43°-AFOV) and 55mm (2"-barrel). World-renowned sharpness and contrast, and very reasonably-priced for Televue eyepieces! Most retail for $100 or less. The 55mm retails for ~$200 and best reserved for slow refractors and SCT's. Most used prices are $70-$80. CLONES: Um...everyone. EVERYONE. I’m only half-joking. Astro-Tech Hi-Grade / Sterling Plossls (recently discontinued)—GREAT bang for the buck. These are 5-element, 1.25”-eyepieces with focal lengths of 4mm, 6mm, 12.5mm, 17mm and 20mm and a 55°-AFOV. A touch soft at the edge (not bad) in fast scopes like 8”+ dobsonians, but very good contrast for the price...which is typically $30-$40 new. Zhumell Z-Series / Orion Edge-On Planetary Series—Another “unsung” series. All 1.25”-barrels, 55°-AFO V, but their 7-element design and 20mm of eye relief sets them apart. Decent performance even in fast scopes. Focal lengths of 3mm, 5mm, 6mm, 9mm, 12.5mm and 14.5mm. Prices very reasonable, all under $100 new. “Musayama Clones” (most discontinued, but often pop up in the used markets)--Orion Ultrascopic, Celestron Ultima (not the LX variety), Parks Gold Series, Baader Eudioscopic, Antares Elite, Takahashi LE. 5-element eyepieces with 52°-AFOV with eye-relief on par with similar focal-length plossls, but with better edge correction vs. most plossls. Common focal lengths are 3.5mm, 5mm, 7.5mm, 10mm, 15mm, 20mm, 25mm, 30mm, 35mm (49°-AFOV). Retail ~$100 new. Used Orion Ultrascopics and Celestron Ultimas go quick on Astromart or Cloudy Nights Classifieds. The “parent series”--the original Musayama design from mid- 1980s Japan--is HIGHLY prized in used markets and often go for $300+/each. Vixen SLV Eyepieces—all 1.25”-eyepieces with a plossl-like 50°-AFOV, but 20mm of eye relief across all focal lengths, which are 2.5mm, 4mm, 5mm, 6mm, 9mm, 12mm, 15mm, 20mm and 25mm. Retail ~ $170/each. Initial reviews very promising. Vernonscope Brandons--Very similar to plossl design, ~45°-AFOV. Only single-coated elements vs. multi- coatings by competition, but highly-polished and renowned for planetary and double-star performance. 48mm (2") is a rare bird. Zeiss Abbe Orthos (I and II)--45° AFOV. The ultimate planetary eyepiece and the standard by which all other planetary eyepieces are judged. Both series have unbeatable coatings, contrast, transmission, on-axis sharpness...and cost!! They're legendary, discontinued, come from a prestigious firm, and are thus EXTREMELY expensive; selling from $600 - $1,000 on used market. The 34mm (ZAO-I only) can net $2,000 itself. Even the walnut carrying case goes for $200 on the used market! Pentax XO and Pentax SMC orthoscopics: very close to ZAO's; insanely sharp on-axis, world-renowned planetary performance...oh, yeah, and PRICE. Baader Genuine Orthos (~$85 retail), University Optics Super Abbe ($60) and H.D. Abbe ($95), Hutech (discontinued). All are excellent performers on planets, won’t swallow your mortgage payment, and are a worthy addition to your eyepiece stable if you have a driven scope. II. 60°-CLASS EYEPIECES: Televue DeLite--62°-AFOV, 20mm ER, 1.25"-only barrels. Focal lengths (some new for 2016): 6mm, 7mm, 9mm, 11mm, 15mm and 18.2mm. Retail around $250 / $200 used. Outstanding on- and off-axis sharpness. Identical eye relief and barrel size as Televue Radians (60°-AFOV, recently discontinued), but with neutral color- tone. Aftermarket Radians are good alternatives to DeLites. Orion Epic II / AstroTech Paradigms / Celestron X-Cel LX / Meade 5000 HD-60°, to name a few. In the 60° class, there's a gazillion Chinese-manufactured eyepieces that appear to be from the same clone batch of 1.25”-eyepieces with focal lengths of 5mm, 8mm, 12mm, 15mm, 18mm and 25mm with around 15mm of eye relief. Most are reasonably-priced around $65 new and are decent performers even in fast (<f/6) scopes. One of the better bargains out there, especially if you're looking to upgrade from the plossls supplied with your new telescope, but are on a budget. Meade 5000 Super Plossl –These are labeled as plossls, but they are in reality 5-element, 60° eyepieces. The Meade 5K Super Plossls have five elements, vs. the 5000 HD-60's six. These are plossl-like eyepieces stretched too far beyond their customary 50° design, and it unfortunately shows in fast scopes. But they're perfectly acceptable in slow scopes like f/8 refractors and f/10 SCT's. III. SUPERWIDE AND ULTRAWIDE EYEPIECES: Televue Ethos (2"/1.25"-barrel, 100°-AFOV unless otherwise noted): 3.7mm (110°-AFOV), 4.7mm (110°-AFOV), 6mm, 8mm, 10mm, 13mm, 17mm (2"), and 21mm (2"). Mind- blowing AFOV, extremely sharp on- and off-axis, excellent contrast. 21mm is VERY expensive, ~$900 retail. All others retail around $600, except $750 for 17mm. CLONES: Explore Scientific 100° Series ("ES-100"): all 2"-barrels: 5.5mm, 9mm, 14mm, 20mm, 25mm. Eye relief listed as 12-14.5mm, but eye lens recessed more than Televue Ethos. 14mm is VERY close in performance to 13mm Ethos, all others just a tiny bit short. Excellent values, particularly the 14ES-100 and 20ES-100. William Optics 101°-Series and Lunt Engineering--100° eyepieces are essentially the same as the ES-100’s. Televue Naglers (82° Ultrawide) Type 1 (discontinued): 4.8mm (short eye relief!!), 7mm, 9mm, 11mm (rare!), 13mm. Type 2 (discontinued): 12mm, 16mm, 20mm (heavy!!). Type 4: 12mm, 17mm, 22mm. Eye relief is 17-19mm; eyeglass-friendly. 12mm and 17mm are dual 2"/1.25"- barrels; 22mm is 2"-only. Prices $375 - $500 new / $250 - $400 used Type 5: 16mm (only 10mm eye relief), 20mm (12mm ER), 26mm (17mm ER), and 31mm (19mm ER). 16mm has 1.25"-barrel; all others 2". Prices $350 - $600 new / $250 - $500 used Type 6: 2,5mm, 3.5mm, 5mm, 7mm, 9mm, 11mm, 13mm, all 1.25"-only barrels w/12mm eye relief. All very compact and light, priced in the low-$300s. The 13T6 is a jack-of-all-trades eyepiece for dobs. Common Nagler Clones: Explore Scientific 82°-Series / Meade 5000 UWA / Celestron Luminos / Celestron Axiom LX (recently discontinued). Focal lengths: 4.7mm, 5.5mm (Meade only), 6.7mm, 8.8mm, 11mm (ES only), 14mm, 18mm, 24mm (2”) and 30mm (2”). Celestron Axiom LX's are probably the same focal lengths from the 6.7mm on up, but marketed as "7mm", "10mm", "15mm", "19mm", "23mm", and "31mm". Axioim LX superior to Luminos series; the latter series has issues with edge-of-field brightening. Most are like having 90% of a Nagler’s performance for just 35-50% of their Nagler counterpart.